In a mobile communications system, a backhaul network can separate a radio base station into base band units, also referred to as main units (MU), and remote radio units (RRUs). An example conventional mobile communications system is shown in FIG. 1 and includes three RRUs 100 that are connected by a network 120 to one or more MUs 130. The MUs 130 can be centralized and located tens of kilometers from the RRUs 100, which are placed close to radio antennas 102 which may be mounted on masts/towers. Colocating the RRUs 100 and serviced antennas 102 minimizes/reduces feeder and jumper losses, which can increase uplink capacity of mobile services by the communications system.
This main-remote system architecture is gaining significant interest and has some clear advantages, e.g. when the RRUs 100 are installed close to the antennas 102. Example products that can be used in a main-remote system architecture are the Antenna-Integrated Radio (AIR) products and the Micro/Small RRU products manufactured by Telefonaktiebolaget L M Ericsson.
The interface between the MUs 130 and RRUs 100 is typically an optical interface carrying NRZ signals, which are formed by sampling I-Q air waveform signals received from antennas 102. Sampling the air waveform reduces complexity of the RRUs 100, but leads to very high bitrates that are output through the optical interface from each RRU 100, e.g., on the order of 1.25 Gbps per antenna 102.
A typical RRU location has many RRUs 100 and antennas 102. To reduce the required number of fibers that communicatively connect all RRUs 100 at the RRU location to the MUs 130, the RRUs 100 are daisy-chained to generate a multiplexed digital output signal of up to 10 Gbps, which is presently the highest rate supported by the Common Public Radio Interface (CPRI) protocol generally used between a RRU and MU. The digital output from the RRUs 100 can therefore pass through a time division multiplexer 110 that time division multiplexes the digital output signals from each of the RRUs 100 into different time slots for a same optical wavelength provided to the network 120.
The very high data rates output from a RRU location, presently up to around 10 Gbps, necessitate the use of expensive electro-optics equipment to interconnect the RRUs 100 and MUs 130. Moreover, because the RRUs 100 are daisy chained while using the CPRI protocol, the antennas 102 become “invisible” to the metro optical network because the network 120 sees an aggregation of the antennas 102 on any one of the optical wavelengths.
For mobile backhaul, it is desirable to re-use low cost passive optical network (PON) technology which is presently available at high volumes (e.g., millions/year) for use in fixed access networks. However, 10 Gbps PON technology is presently available at substantially lower volumes for fixed access networks and, therefore, is relatively expensive, especially when deployed in large numbers in very dense networks.
The approaches and presently recognized problems described above in this section could be pursued, but are not necessarily approaches and/or problems that have been previously conceived or pursued. Therefore, unless otherwise clearly indicated herein, the approaches and problems described above in this section are not prior art to claims in this application and any application claiming priority from this application and are not admitted to be prior art by inclusion in this section